PHOENIX – Rickey Henderson’s 40-year-old right knee just won’t heal completely, so the Mets are placing the left fielder in the disabled list.

“It’s not progressing as much as we would like it to,” GM Steve Phillips said of the knee.

This assignment on the DL will be the third for a Met regular this season, and that doesn’t include all the time Bobby Bonilla has missed with his injured left knee. Rick Reed was on the DL with a torn calf muscle and Mike Piazza went on in April after spraining his right knee.

Henderson strained his right knee going after a ball in the outfield Sunday, May 2, and hasn’t played since. Although he took batting practice yesterday, Henderson cannot run on the knee and it wasn’t likely that he would be able to in the next five or six days, so the Mets decided to put him on the DL and call up an as-yet-undetermined player form Triple-A Norfolk.

But it will be Roger Cedeno who will get most of the playing time. Cedeno has been playing well as a fill-in for Henderson, Bonilla and Brian McRae, hitting .321 with a team-high eight stolen bases. He has made it into all but one of the Mets’ 28 games.

“That’s a good situation,” Bobby Valentine said of Cedeno’s ability to take over.

It is not believed that Henderson’s injury is any more serious than what was originally diagnosed and the team had no plans to send him back to New York for an MRI. After an examination by trainer Fred Hina yesterday, the determination was simply that Rickey needed at least five or six more days to rest the knee.

“Actually, 10 days would be rushing it,” Valentine said. “So, let’s not rush it. There’s no sense in forcing the issue and then losing him for three weeks.”

That means he wouldn’t have been able to play until at least the weekend series in Philadelphia, but that’s on synthetic turf, so he wouldn’t have played there anyway. Therefore, the Mets will make it retroactive to Monday, which means Henderson will be able to come off the DL Tuesday, May 18 against the Brewers at Shea.

“I have no problem with it,” Henderson said. “I would have a problem if I thought I’d be ready, but that’s not the case. I told them, if you need help you’ve got to do it. I think they were worried about me rushing to get back and not letting it heal.”

Henderson said he tried to run before yesterday’s game and was unable to do it, and said the only way he could have played was if there were a designated runner, or if all they needed was a home run.

“Maybe if we had a tag team at home plate and then the guy’s got to run,” he said. “Or maybe if it’s do-or-die, call me up then.”

Henderson, who has had his share of leg injuries over his career, said he has never had this one before so he wasn’t sure what to make of it. He said the original swelling in the knee has dissipated, but it has moved down to his foot.

As for what form that help might come in, the Mets had not made a decision prior to last night’s game as to whom would get the call up. With Bobby Bonilla struggling with a big brace and trying to overcome a knee injury of his own, and the same with McRae, the Mets are likely to call up an outfielder such as Terrence Long.

But with three games coming up in the thin air of Denver, the Mets were considering bringing up a pitcher to fill in just in case things get out of hand there. All Valentine would say is that they would not call up a catcher. There has been some consideration in that direction because it would mean Todd Pratt could get more chances to pinch hit if there were another catcher behind him. But so far that plan is not on the front burner.